Especially for long term storage, I wonder how they should be preserved - some straps give up and start scaling, I guess the upper/hide parts usually survive, but the lower, soft leather parts are just hard to preserve

Is sealing them a good idea?

Some plastics stain leather/plastics too, so I guess it's also a slippery slope, but ziplocking it seems like a good idea to me

 

I guess the climate may be something to consider. If it's really humid, or really dry, this will affect the leather. The best solution for watches and things that go bang, is a climate controlled safe. I live in a pretty temperate region. My watches are kept in a safe in plastic sleeves then put in another plastic storage container in the safe. Lexol cleaner is great for cleaning and preserving leather. If I wear a strap in the summer and it gets wet from perspiration, i let it sit out and dry for a day. Then I wipe the strap down and put it in the plastic sleeve. Seems a little OCD, but my leather straps last for years.

 

I think the quality of the strap makes a difference too, and that doesn’t necessarily mean the most expensive. I’ve had expensive straps that stain easily due to the lining leather being inferior.
Without doubt ( and also recommended for shoes ) allow the leather to dry out thoroughly before storing

 

First, you need this stuff:

https://fiebing.com/product/leather-lotion/

With a damp cloth, wipe the dirt off best you can and dry well
Apply Leather Lotion, rub it in with your fingers, and dry well. Usually does not darken but test first. If it does look a bit darker, don't panic, it will lighten as dries.
Store in a cool place, not too wet and not too dry.
Before reusing, if the strap has sat for a looong time, apply more Leather Lotion.